Hardware suffers physical degradation and the resulting random failure rates can be described numerically using well established methods of statistical reliability. In contrast, software does not degrade physically, and all failures result from systematic factors in its construction and use. It is not currently widely accepted that conventional reliability analysis can be applied to systematic behavior. Therefore, the standard recognizes that a quantitative demonstration that the target failure measures for safety integrity levels in tables 2 and 3 of IEC 61508-1 have been met is in general possible only for random hardware failures (see note 8 of 7.6.2.9 of IEC 61508-1). The effectiveness of the measures and precautions used to meet the target failure measures for systematic safety integrity (and specifically software) generally needs to be assessed qualitatively.
However, despite the above difficulties, tables 2 and 3 of IEC 61508-1 provide a valuable framework for comparing different levels of achievement of systematic safety integrity.
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